The present invention relates to a system for starting a welding arc and, more particularly, to a system for starting a welding arc without using AC voltage and without having a diode in the circuit of the welding power supply.
In arc welding, the voltage of the open-circuited, high current, welding power supply is always much lower than the breakdown voltage across the electrode-to-workpiece gap. Therefore, additional means must be provided for initiating the arc.
These means, typically, include moving the electrode to touch the workpiece, or applying a high starting voltage across the gap for a short period of time. In many welding applications, touching has serious drawbacks such as weld contamination, difficult implementation in automated welding situations, and possible burn-through of the work. Application of a high AC voltage solves these problems, because the AC voltage can be tailored to break down the gaseous atmosphere in the gap and form a conducting plasma which will sustain an arc from the lower-voltage DC power supply.
The AC starting voltage does have a significant disadvantage in some applications; i.e., it radiates energy as RF interference for electrical circuits that may be either on the workpiece or controlling the welding machine. For this reason, the use of high voltage DC to initiate the gap has been explored.
U.S. Pat. No. 3,571,558 by John A. Hogan, Jr., shows an arc starting system where a high voltage output of a step-up transformer is half-wave rectified by a diode. Once gap breakdown occurs, the welding power supply begins to conduct through the diode. In order to protect this diode, which must support the heavy welding current, the patent also provides sensing and bypass relays to short circuit the diode once welding current begins to flow.
U.S. Pat. No. 4,672,175 by Robert W. Nivens shows another arc starter where the high voltage power supply is powered by the welding power supply and which uses an arc detector to operate a relay to bypass a diode in series with the welding power supply.
Each of the aforementioned patents recognize that the diode in series with the welding power supply must carry the heavy welding current, and try to minimize the effect on this diode by bypassing it as quickly as possible. This invention also recognizes the same problem, but solves it by eliminating the series diode and connecting the welding power supply directly across the gap.